Generally, a VOR system includes ground stations and receivers. The ground stations transmit navigation guidance signals used by aircraft in flight. The aircraft includes a VOR receiver for receiving the signals transmitted by the ground stations. The VOR system may be described as a classical VOR (CVOR) or a Doppler VOR (DVOR) system.
In CVOR, the ground station transmits a rotating cardioid shaped antenna horizontal radiation pattern at thirty revolutions per second and a fixed omni-directional carrier pattern. The receiver obtains a carrier that is amplitude modulated with a 30 Hz sine wave, the phase of which is dependent on the azimuth position of the aircraft in relation to the ground station. In order to use the bearing information, the ground station provides a reference by amplitude modulating the carrier with a sub-carrier of 9960 Hz, which is, in turn, frequency modulated by a 30 Hz sine wave with a deviation of ±480 Hz. The phase of the 30 Hz frequency modulation is independent of azimuth. The aircraft bearing in relation to the ground station is calculated by taking the phase difference of the two 30 Hz sine waves.
DVOR employs two fundamental principles: the Doppler effect for generating frequency modulated (FM) and bearing information, and a wide aperture antenna array for minimizing the effects of multipath propagation. To maintain compatibility with CVOR receivers, DVOR ground stations radiate signals with the same frequency spectrum as the CVOR ground stations, but the azimuth-dependent information is contained in the phase of the frequency modulated signal. In DVOR, the carrier with a 30 Hz amplitude modulation is radiated from an omni-directional antenna and is the reference signal. The direction dependent signal is generated in space by rotating the radiated 9960 Hz sidebands on a circle.
The circular motion is electronically simulated by a number of antennas equally spaced around the circle, which are sequentially fed with radio frequency (RF) energy so that a continuous movement of the radiating source is achieved. The DVOR receiver sees a Doppler shift of sideband frequencies deviating ±480 Hz thirty times a second. The DVOR system may be a single sideband DVOR, a doubled sideband DVOR, or an alternating double sideband DVOR system.
The European Organisation for Civil Aviation Electronics (EUROCAE) has specified minimum performance requirements suitable for airborne VOR receivers. These specifications can be found in EUROCAE document ED-22B (January 1988), which is hereby incorporated by reference in its entirety. The ED-22B document includes compatibility requirements with DVOR ground stations. (See, Chapter 3, paragraph 3.2.2.2.)
To verify that the VOR receiver meets the compatibility requirements with VOR ground stations, the ED-22B document provides test procedures. (See, Chapter 5, paragraph 5.2.3.3.) As described in the ED-22B document, DVOR signal generators are not currently available, so the document provides a list of the equipment needed for each of the tests and a diagram showing how to arrange the equipment to perform the test. For example, the first test requires an oscilloscope, an RF signal generator, an audio frequency (AF) VOR signal generator (with separate 30 Hz and 9960 Hz outputs), an AF signal generator, an amplitude modulating (AM) modulator, and a deviation indicator or microammeter of equivalent resistance. FIG. 5-2 of the ED-22B document depicts the arrangement of the equipment to simulate the DVOR signal as required by the first test.
Due to the numerous pieces of equipment needed to perform the tests, the test environment for testing the VOR receiver is less than desirable. In general, this test environment may have issues with calibration, accuracy, and repeatability. The tests may fail, not because of a VOR receiver failure, but because of a problem in the test setup. For example, if one of the pieces of equipment is not calibrated properly, the VOR signal may not be properly simulated for the test. Other problems may also occur due to, for example, as poor connections between equipment, incompatibility between equipment, and equipment failures.
Thus, it would be beneficial to have an improved method of generating VOR signals for testing a VOR receiver.